Conventionally, an epoxy resin coating exhibiting excellent curability at ordinary temperature has been used for coating large-scale architectures such as vessels, bridges, manufacturing plants, roads and floors, wherein curing by heating is not available.
Also, as a coating composition which is curable during the winter season or in cold regions, a coating composition containing polyisocyanate has been developed and used in the cases where curing at a temperature of 5° C. or lower is required. A tin composition is used as a curing catalyst in polyisocyanate-based coating compositions, and the use of the tin compound is seen as a problem because even a small amount of a tin compound is highly toxic. Therefore, a coating composition which has low toxicity and is curable at low temperatures has been demanded.
A compound containing two or more thiol groups in one molecule reacts more readily to form a cured product by being mixed with epoxy resin, urethane resin and the like, such a compound has been widely used for sealing materials, coatings, adhesives and the like. For example, “Epoxy Resin Review” (Vol. 1, Basic Edition I, published on Nov. 19, 2003) describes at page 204 various polythiol-based curing agents as a curing agent at low temperature.
Meanwhile, as an epoxy resin coating composition, it is desired to employ the formulation of mixing two liquids comprising liquid (A) and liquid (B) just before coating due to ease in handling.
However, in the case where a conventional polythiol-based epoxy curing agent is used as a curing agent for an epoxy resin coating composition, when polythiol is added to liquid (A) containing epoxy resin, it decreases storage stability due to high reactivity between epoxy resin and polythiol. When conventional polythiol is added to liquid (B) containing a curing assistant, thiol is activated by the tertiary amine used as a curing assistant and generates a disulfide bond by reacting with oxygen in air, which often results in forming a skim during the storage of liquid (B).
JP-A-H06-116515 (Patent Document 1) discloses an antifouling coating composition containing epoxy resin and a mercapto carboxylic acid ester curing agent for the use as an antifouling paint in sea water used for vessels and the like. Patent Document 1 describes formulation of obtaining a coating by mixing liquid (A) containing epoxy resin, a pigment and the like and liquid (B) containing a mercapto carboxylic acid ester curing agent and an amine curing agent. However, the mercapto carboxylic acid ester disclosed therein is a compound in which a thiol group is bonded to primary carbon, unlike the branched compound containing a thiol group (P) used in the present invention, and therefore has a problem in storage stability of the solution containing a mercapto carboxylic acid ester curing agent.
JP-A-2002-527552 (WO 00/22025) (Patent Document 2) discloses a coating composition containing epoxy resin, a curing agent containing a thiol group and a catalyst for curing, which has attained a suitable pot life and a suitable curing time by using a specific solvent for the mixture of epoxy resin, a thiol compound and a curing assistant. However, Patent Document 2 does not describe a branched compound containing a thiol group (P) used in the present invention, and the storage stability of the curing agent containing a thiol group either. Furthermore, though Patent Document 2 teaches storing a catalyst separately from resin, it does not teach storing a catalyst being mixed with a curing agent. Therefore, it is not clear how the curing agent containing epoxy resin, a catalyst and a thiol group described in Patent Document 2 can be stored as being two liquids.
JP-A-08-176500 (Patent Document 3) discloses a method for applying a rapidly curable coating composition, which comprises applying a composition containing an epoxy compound and a polymercapto compound in a specific ratio on a substrate to be cured, but Patent Document 3 does not describe a branched compound containing a thiol group (P) used in the present invention. In the coating method described in Patent Document 3, it is necessary to apply a curing catalyst such as secondary amine and tertiary amine in the form of mist or steam, which makes the operation complicated and raises a problem in terms of the work environment such as an irritating odor of the curing assistant.